async/await continuation

Question

在下面的代碼中，我需要並行執行三個Get ...方法。當Get ...方法完成時，我需要立即調用Save ...方法。注意保存...將事物作爲參數。 所有獲取和保存方法必須在DoStuffAsync返回之前完成。

我的猜測是我需要Get ...方法的延續，但我不知道如何構造它。

protected async void DoStuffAsync() 
{ 
    SomeThing thing = new SomeThing { ID = 5 }; 
    SomeRepository rep = new SomeRepository(); 

    // We need to run all three Get... methods in parallel 
    // As soon as a Get... method completes we need to save the result to the correct property on thing and call the Save... method . 

    var getRed = rep.GetRedAsync().ContinueWith<Task<string>>(async x => { thing.Color1 = x.Result; await rep.SaveRedAsync(thing); return x; }); // does not compile 
    var getBlue = rep.GetBlueAsync(); 
    var getGreen = rep.GetGreenAsync(); 

    string red = await getRed.Result; // this is not good because getBlue may finish before getRed. We want dont want to wait on getRed before calling SaveBlue 
    await rep.SaveRedAsync(thing); 
    var b = await getBlue; 
    var c = await getGreen; 

    // thing must be fully initialized before DoStuffAsync returns 

    } 


public class SomeThing 
{ 
    public int ID { get; set; } 
    public string Color1 { get; set; } 
    public string Color2 { get; set; } 
    public string Color3 { get; set; } 
} 

public class SomeRepository 
{ 
    public async Task<string> GetRedAsync() 
    { 
     return await Task.Run(() => "red"); 
    } 

    public async Task<string> GetBlueAsync() 
    { 
     return await Task.Run(() => "blue"); 
    } 

    public async Task<string> GetGreenAsync() 
    { 
     return await Task.Run(() => "green"); 
    } 

    public async Task SaveRedAsync(SomeThing thing) 
    { 
     // We need thing.ID here as well as other properties 
     await Task.Delay(1); 
    } 

    public async Task SaveBlueAsync(SomeThing thing) 
    { 
     await Task.Delay(1); 
    } 

    public async Task SaveGreenAsync(SomeThing thing) 
    { 
     await Task.Delay(1); 
    } 
} 

Answer 1

我不會混ContinueWith和await和而使用直接的async拉姆達：

protected async Task DoStuffAsync() 
{ 
    SomeThing thing = new SomeThing { ID = 5 }; 
    SomeRepository rep = new SomeRepository(); 

    // We need to run all three Get... methods in parallel 
    // As soon as a Get... method completes we need to save the result to the correct property on thing and call the Save... method . 

    Func<Task<X>> getRedFunc = async() => 
    { 
     var result = await rep.GetRedAsync(); 
     thing.Color1 = result; 
     await rep.SaveRedAsync(thing); 
     return result; 
    }; 

    var getRed = getRedFunc(); 

    var getBlue = rep.GetBlueAsync(); 
    var getGreen = rep.GetGreenAsync(); 

    await Task.WhenAll(getRed, getBlue, getGreen); 
} 

另外，不要使用async void方法什麼，但事件處理程序。你將無法觀察到像這樣的方法的完成，或者處理可能拋出的異常。

Answer 2

好了，你可以明確使用ContinueWith - 或者你可以中斷每個「獲取和保存」到一個單獨的異步方法或異步拉姆達。例如：

async Task GetAndSaveRedAsync(SomeThing thing, SomeRepository rep) 
{ 
    var red = await rep.GetRedAsync(); 
    thing.Red = red; 
    await SaveRedAsync(red); 
    // Return red if you want (change the return type to Task<string>) 
} 

// Ditto for the others 

然後：

protected async void DoStuffAsync() 
{ 
    SomeThing thing = new SomeThing { ID = 5 }; 
    SomeRepository rep = new SomeRepository(); 

    var handleRed = GetAndSaveRedAsync(thing, rep); 
    var handleBlue = GetAndSaveBlueAsync(thing, rep); 
    var handleYellow = GetAndSaveYellowAsync(thing, rep); 

    // Or use Task.WhenAll 
    await handleRed; 
    await handleBlue; 
    await handleYellow; 
} 

Answer 3

您可以嘗試並行框架：

using System.Threading; 

using System.Threading.Tasks; 

Func<Task<string>>[] functions = { rep.GetRedAsync, rep.GetBlueAsync, rep.GetGreenAsync }; 
Var[] GetArray = new Var[functions.Length] 
int i=0; 
Parallel.ForEach (var function in functions) 
{ 
      GetArray[i++]=function(); 

} 

注意：要求.Net 4